A backlight, as a light source device of a display device such as a liquid crystal display device, uses a light source device comprising one or a plurality of discharge tubes and a reflector. The discharge tube is a cold cathode tube, in which mercury is contained in a lean gas (such as Ar and Ne) and a fluorescent material is coated on the tube wall. Electrodes are provided in both ends of the discharge tube. The discharge tube is supported by the reflector with support members which are arranged at positions near the electrodes of the discharge tube. The backlight also has a light guide plate, and the light source device is arranged on the side of the light guide plate.
When the high voltage is applied to the electrodes of the discharge tube, electrons are emitted by the electrodes and impinge against the mercury gas in the discharge tube. The mercury gas produces ultraviolet light, which impinges against the fluorescent material in the discharge tube to emit visible light. As the amount of the mercury gas in the discharge tube is reduced, the amount of emitted light is reduced and an operational life thereof ends. Usually, a sufficient amount of mercury gas is sealed in the discharge tube to enable the discharge tube to have a long life, but there are discharge tubes having extremely short operational lives.